1 Electrocardiography – Normal 6 Faisal I. Mohammed, MD, PhD

Objectives Recognize the normal ECG tracing Calculate the heart rate Determine the rhythm Calculate the length of intervals and determine the segments deflections Draw the Hexagonal axis of the ECG Find the mean electrical axis of QRS (Ventricular depolarization)

Principles of Vectorial Analysis of EKG’s The current in the heart flows from the area of depolarization to the polarized areas, and the electrical potential generated can be represented by a vector, with the arrowhead pointing in the positive direction. The length of the vector is proportional to the voltage of the potential. The generated potential at any instance can be represented by an instantaneous mean vector. The normal mean QRS vector is 60 o (-30◦ - 110◦)

_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ Mean Vector Through the Partially Depolarized Heart +

5 Einthoven’s triangle and law  + +  + 

Principles of Vectorial Analysis of EKG’s (cont’d) Axes of the Three Bipolar and Augmented Leads + _ _ _ o 120 o 0o0o I I III II III II + _ -30 o aVL + _ 210 o aVR + _ aVF 90 o

Axes of the Unipolar Limb Leads I aVL aVR aVF -

Principles of Vectorial Analysis of EKG’s (cont’d) The axis of lead I is zero degrees because the electrodes lie in the horizontal direction on each of the arms. The axis of lead II is +60 degrees because the right arm connects to the torso in the top right corner, and left leg connects to the torso in the bottom left corner. The axis of lead III is 120 degrees.

Principles of Vectorial Analysis of EKG’s (cont’d)

11 Principles of Vectorial Analysis of EKG’s (cont’d) In figure B, the depolarization vector is large because half of the ventricle is depolarized. Lead II should be largest voltage when compared to I and III when the mean vector is 60 o. In figure C, left side is slower to depolarize. In figure D, the last part to depolarize is near the left base of the heart which gives a negative vector (S wave). Q wave is present if the left side of the septum depolarizes first.

The T Wave (Ventricular Repolarization) First area to repolarize is near the apex of the heart. Last areas, in general, to depolarize are the first to repolarize. Repolarized areas will have a + charge first; therefore, a + net vector occurs and a positive T wave

Atrial Depolarization (P-Wave) and Atrial Repolarization (Atrial T Wave) Atrial depolarization begins at sinus node and spreads toward A-V node. This should give a + vector in leads I, II, and III. Atrial repolarization can’t be seen because it is masked by QRS complex. Atrial depolarization is slower than in ventricles, so first area to depolarize is also the first to repolarize. This gives a negative atrial repolarization wave in leads I, II, and III

Vectorcardiogram Note zero reference point, number 5, is point of full depolarization. This traces vectors throughout cardiac cycle. When half of the ventricle is depolarized, vector is largest.

I II III _ I _ o 0o0o I III Plot of the Mean Electrical Axis of the Heart from Two Electrocardiographic Leads 60 o -60 o 180 o

Lead I + Lead I  aVF  aVF + 0◦ 180◦ +90◦ 90◦ NORMAL MEAN ELECTRICAL AXIS OF QRS From 0 to +90 RIGHT AXIS DEVIATION OF QRS From +90 to SEVERE RIGHT OR LEFT AXIS DEVIATION OF QRS From 180 to 360 (-90) LEFT AXIS DEVIATION OF QRS

19 Heart Rate Calculation R-R interval = 0.83 sec Heart rate = (60 sec)/(0.83 sec) = 72 beats/min min beat

20 ECG Calculations

21 ECG Calculations

22 Determine regularity Look at the R-R distances (using a caliper or markings on a pen or paper). Regular (are they equidistant apart)? Occasionally irregular? Regularly irregular? Irregularly irregular? Interpretation? Regular RR

23 ECG Deflection Waves (Pacemaker) Atrial repolarization

24 ECG Deflection Waves 1st Degree Heart Block = P-Q interval longer than 0.2 seconds. 60 seconds ÷ 0.8 seconds = resting heart rate of 75 beats/minute

25 ECG Deflection Wave Irregularities Enlarged QRS = Hypertrophy of ventricles

26 ECG Deflection Wave Irregularities Prolonged QT Interval = Repolarization abnormalities increase chances of ventricular arrhythmias.

27 ECG Deflection Wave Irregularities Elevated T wave : Hyperkalemia

28 ECG Deflection Wave Irregularities Flat T wave : Hypokalemia or ischemia

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